Question

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How do I check to see if an object has an attribute in Javascript?

I have a Javascript object defined as following:

var mmap = new Object();

mmap['Q'] = 1;
mmap['Z'] = 0;
mmap['L'] = 7;
...

How to check whether this map has a value for a given key (for example ‘X’)? Does .hasOwnProperty() get into play?

Answer 1

if ('X' in mmap)
{
    // ...
}

Here is an example on JSFiddle.

hasOwnProperty is also valid, but using in is much more painless. The only difference is that in returns prototype properties, whereas hasOwnProperty does not.